1. Field of the Invention
The present invention relates to mobile ad hoc networks, and particularly to medium access control by nodes in such networks.
2. Discussion of the Known Art
Increasing the spectral efficiency of a wireless communications network is a constant challenge. Most military and commercial applications require wireless networks to operate reliably in dynamically changing environments while maintaining high rates of data transmission over established links. Due to limitations in hardware capability at the physical (PHY) layer, wireless communications networks typically operate under an interference avoidance protocol. Such a protocol presents a major hurdle to increasing spectral efficiency, however.
So-called multiuser detection (MUD) techniques have improved spectral efficiency by increasing spectrum reuse within a given network, and most MUD research to date is directed at the PHY network layer. MUD enabled medium access control (MAC) for mobile ad hoc networks (MANETs) has appeared in the literature only recently, and focuses on fairness of medium access. See, e.g., K. Kusume, et al., “Medium Access in Spread-Spectrum Ad Hoc Networks with Multiuser Detection,” Eurasip Journal on Advances in Signal Processing” (2009); and J. Zhang, et al., “Multiuser Detection Based MAC Design for Ad Hoc Networks,” IEEE Transactions On Wireless Communications, vol. 8, no. 4 (April 2009) at pages 1836-46; both of which publications are incorporated by reference. Moreover, most MAC layer studies rely on the Global Positioning System (GPS) for synchronization in MANETs.
In Kusume, et al., supra, the authors propose a multiple access scheme in MUD enabled systems to resolve the so-called hidden-node problem. A timing frame is proposed having four zones, two of which called ANN (announcement) and OBJ (objection) serve a function similar to that of the known RTS/CTS (request-to-send/clear-to-send signaling protocol defined in the IEEE 802.11 wireless standard. In Zhang, et al., supra, the authors propose a two-stage RTS/CTS scheduling scheme to achieve a distributed spread spectrum code assignment among the network nodes.
In addition to fairness of channel access, a MAC scheme for use in tactical situations must consider many other requirements, for example, synchronization without using GPS or infrastructure, overhead efficiency, optimization of dynamic resource allocation, support for dense topologies, mobility, scalability, and quality of service (QoS). Little work has been reported on system designs or implementations that address these concerns, however. Making the task even more challenging is the fact that some of these requirements conflict with one another. For example, high channel efficiency (ratio of time for application data vs. time for non-application data) often conflicts with QoS, and contention based RTS/CTS access protocols cannot support QoS, especially in densely deployed networks.
Contention based access approaches also face the challenge of having to balance between high channel efficiency which usually requires relatively short time periods for RTS/CTS signaling and longer time periods for DATA transmission, and scalability which requires relatively long time periods for RTS/CTS signaling to resolve access collisions in densely deployed networks. Any network timing scheme that does not consider and balance such requirements would have limited utility.
A program developed under the Defense Advanced Research Projects Agency (DARPA) and known as the DARPA Interference Multiple Access (DIMA) Communications program, involves a real time prototype communication system that exploits MUD techniques to allow multiple simultaneous transmissions, thus resulting in higher spectral efficiency. DIMA operates in an ad hoc fashion without a need for infrastructure or central controllers, or relying on GPS. Moreover, DIMA has demonstrated more than a threefold improvement in spectral efficiency compared to IEEE 802.11. At low signal to noise ratios (SNRs), a DIMA radio is known to have achieved a packet error rate (PER) of less than one percent. See, Y. Eisenberg, et al., “MUD Enabled Media Access Control for High Capacity, Low-latency Spread Spectrum Communications,” IEEE MILCOM, Orlando, Fla. (October 2007), which publication is incorporated by reference.